Abstract: Experimental study of transmission line was carried out in order to clarify the resonance galloping mechanism. To this end, a 220 kV three-phase transmission line project in Hunan province was taken as a prototype, and then the test model of tower-line system was designed on the basis of dynamic similarity principle. Besides, a non-contact electromagnetic excitation system was developed in order to retain the dynamic characteristics of transmission line to the maximum. Under various excitation conditions (single-phase excitation in-plane, three-phase excitation in-plane and three-phase excitation out-of-plane), the forced vibration characteristics of transmission line were analyzed. The results show that the system resonance is the main mechanism resulting in the galloping of transmission line. The excitation frequencies inducing galloping are all close to the natural frequencies of each order of the system, and the vibration modes are consistent. Also, there are obvious coupled vibrations between tower-lines and phase-phase. However, the non-excitation phase may also result in a large galloping, and the amplitude sometimes even exceeds the excitation phase. And there is an obvious dynamic vibration absorption phenomenon between phases. Obviously, the vibration form of the three-phase transmission line in resonance state is very complicated. From the time history perspective, there are synchronous, reverse synchronous and delayed synchronous vibrations between phases, and the same-period beat vibration or different-period beat vibration often occurs. When different-period beat vibration occurs, the amplitude changes one after another, and energy is transferred back and forth between phases. From the trajectory perspective, there are straight, elliptic, crescent shaped, figure-eight shaped and chaotic movements, and the trajectory shows a drifting phenomenon because of the random disturbance.